1. Field of the Invention
This invention relates to acid stimulation of subterranean formations to increase the formation permeability for increased hydrocarbon production from hydrocarbon bearing formations having inconsistent permeabilities. More specifically, this invention relates to a method of re-directing formation-stimulating fluids to target production zones and diverting the stimulation fluids away from high permeability or undamaged zones by temporarily blocking high permeability zones.
2. Background of the Invention
Matrix acidizing, the process of injecting a formation stimulation fluid such as acid or other acid-forming materials that may react with minerals in the formation to increase the formation permeability, is a common method used to stimulate the formation. This process enhances production of the hydrocarbon product from a hydrocarbon producing formation. To obtain the maximum benefits of matrix acidizing, it is often desirable to treat the entire productive interval of the formation with the stimulation fluid. In many cases, the high permeability or non-damaged intervals are adjacent to the damaged or low permeability intervals in a productive hydrocarbon formation. As the stimulation fluid is pumped, it will preferentially enter the interval of least resistance. This interval is typically the high permeability or non-damaged zone. As stimulation fluid enters the high permeability or undamaged formation, the stimulation fluid will react with the formation material and open additional flow paths. As a result, the high permeability interval receives most or all of the stimulation while the desired low permeability or damaged zones do not receive the desired stimulation.
The overall success or failure of many of these treatments is often judged by the ability to inject or direct the acid into the damaged or lower permeability zone. In order to re-direct the stimulation fluids from the non-damaged intervals into the damaged intervals, a pressure differential across the high permeability or non-damaged intervals may be created. This pressure differential will typically force the stimulation fluid into new portions of the reservoir that otherwise would not receive the stimulation fluid.
A number of techniques and materials have traditionally been available for diverting the stimulation fluid. One way to achieve diversion is mechanically, by using drillpipe or coil tubing-conveyed tools equipped with mechanical packers. Each individual interval may then be isolated and then stimulated. This process is extremely time consuming and frequently very costly.
In addition to mechanical diverting means, chemical diverting agents such as viscous gels or foams may be used to temporarily block the high permeability interval and divert the stimulation fluids into the desired low permeability or damaged intervals. It is desirable for these viscous gels to be stable at the bottom hole temperature and also to be removable from the formation rapidly after the treatment in order to eliminate any potential damage to the high permeability intervals. One chemical diverting fluid is a gelled hydroxyethylcellulose (HEC) pill. This technique relies upon the viscosity of the pill to influence the injection pressure of the interval it enters. As the pill enters the formation, the viscosity of the pill will restrict the injection of other fluids into this area. As the injection pressure increases within this portion of the interval, other sections of the interval will break down and begin accepting fluid. This technique is severely limited if the temperature of the gelled HEC exceeds 200° F. Above this temperature, the base viscosity and life of the pill is greatly decreased. Another problem seen with gelled HEC is that the blocked zone may be damaged from the polymeric residues left inside the porous media once the acid treatment is completed.
Foams may also be used as a diverting method for acid stimulation. Foams typically are generated through a blend of surfactants and/or a polymer. The addition of the polymer may also cause formation damage, as described above, while the use of nitrogen gas tanks and other associated pumping equipment are typically required for foam used as acid stimulation diverting agent. This may not be practical in many cases, especially for offshore acid treatments, as the operation is often limited by available deck space on the rig or vessel. In addition, foams typically become unstable above 250° F.
Polymer-based gels have often been used as diverting agents in combination with acid stimulation; for instance, HEC or hydropropyl guar gum may be used as diverting agent for acid stimulation in fractured formations. This technique normally combines hydraulic fracturing with the acid treatment, which is significantly different from and more costly than a simple acid treatment. Using a polymer based gel as diverting agent will normally result in formation damage caused by the polymer residue.
Another type of viscous fluid diverting agent used to assist in formation stimulation is a surfactant or surfactant mixture. One such viscoelastic fluid forms micelles. These wormlike micelles are sensitive to hydrocarbons. By utilizing this sensitivity, the fluid may selectively block water-bearing zones while the hydrocarbon-bearing zone is unaffected. However, this viscoelastic surfactant fluid typically cannot discriminate between zones with various permeabilities as long as the zones are hydrocarbon-bearing. Further, unlike polymer based fluids which rely upon filter cake deposition to control leak-off to the formation, viscoelastic surfactant diverting agents control fluid leak-off into the formation through the structure size of the micelles. The micellar based VES fluids usually have high leak-off rates to the formation due to the small size of the wormlike micelles. Rapid weaving and breaking of these structures also limits the ability of the micellar based viscoelastic system to control fluid leak-off. The temperature limitations for such a system is generally around 200° F. due to the low stability of micellar structure.
What is needed is a method for diverting the stimulation fluid from high permeability zones to desired low permeability zones. This method should preferably use a composition that does not damage the formation, and is easily removed from the formation.